@Oakmeister 

May I know how do you know that the secondary NDS server does not work? Generally speaking, the secondary DNS server is a backup; it won't take effect until the primary DNS falls down. 

You can disable the primary DNS server and verify whether the secondary one works. 

Good day. 

@Kevin_Z 

If I set my PC's DNS server to 192.168.1.1 my PC is no longer able to resolve domain names and therefore I can't view any websites. 
 

Setting my PC DNS to 8.8.8.8 works fine. 
 

Also if l use the NSLOOKUP command and set server to 192.168.1.1 the server doesn't respond.

I know that the secondary is only used in a backup but unfortunately the Deco is issuing itself as the backup but then is not able to forward those DNS requests. this means there is no backup DNS server and no redundancy should the primary DNS go down. 
 

It's not a huge problem but I don't think the Deco should be doing this by default. Either having the same as the primary or a blank secondary DNS server would be preferable? 
 

thanks